Also remove unused migrations test for MySQL.

Since there is no demo or us site anymore we can simplify the structure
somewhat by moving the sandbox one directory up

This removes all the bits and bobs where we referenced South, or added
some code supporting it. Documentation will be updated in a later
commit, together with other Django changes.

oscar_vagrant was confusing.

* Use correct username for MySQL on Travis
  Travis docs say username should be 'travis', not 'root'.

* Fail test_migrations.sh if any command fails
  Previously, a failing (non-zero exit code) command would not lead to
  a non-zero exit code of test_migrations.sh. That meant that failing
  migrations would not fail the Travis job as intended.

This should help protect against faulty migrations.

Also adjust migration test script.

It recreates the databases and runs all migrations from scratch.